The present invention relates to a request control apparatus for controlling requests to data processor stores for use in computer systems.
In a computer using an on-demand-paging processing method, the main store is usually controlled on a page-by-page basis. The absence of the desired piece of information in the main store is referred to as a page fault, which causes an inhibiting factor to normal operation of the processor, i.e., an exception, to be generated. In response to the detection of the generation of such an exception, paging of the absent page into the main store takes place. Thus a page on which the requested piece of information is present is supplied from a secondary store to the main store. In response to this page supply, the execution of the program is resumed from the top of the group of instructions in which the exception was generated.
For a paging technique to be used in the event of a page fault, reference may be made to the published European Patent No. 0217168 AZ.
At the time of resuming the program execution, there arises the need to cancel the partially executed instruction in order to make possible its full reexecution.
Now there is supposed the execution of an instruction which, in a computer using an on-demand-paging processing technique, has a variable-length operand and requires the reading of data on a memory and storing them, either as they or after being operated on, into a memory or a software visible register, or requires storing of the content of a software visible register into a memory. If, in the middle of the execution of such an instruction, an exception due to the aforementioned page fault is generated and if the content of the memory or the software visible memory is rewritten, the execution of the instruction cannot be resumed.
For execution of such an instruction by the prior art, a check request is issued to the beginning and end of the operand. Then, the instruction is processed after confirming that all of the required pages have been supplied to the main store, or supplying the required page or pages according to the check request and confirming this supply.
In this prior art manner of executing an instruction, however, there is the disadvantage that, even in the presence of operand data on the same page, a check request for the end of the operand has to be issued in order to confirm that all the required pages have been supplied to the main store.